#' Plot arithmetic
#'
#' In addition to the `+` operator known in `ggplot2`, `patchwork` defines logic
#' for some of the other operators that aids in building up your plot
#' composition and reduce code-reuse.
#'
#' @details
#' `patchwork` augment the `+` operator from `ggplot2` and allows the user to
#' add full `ggplot` objects together in order to compose them into the same
#' view. The last added plot is always the active one where new geoms etc. are
#' added to. Another operator that is much like it, but not quite, is `-`. It
#' also adds plots together but instead of adding the right hand side to the
#' patchwork defined in the left hand side, it puts the left hand side besides
#' the right hand side in a patchwork. This might sound confusing, but in
#' essence `-` ensures that the right and left side are put in the same nesting
#' level (`+` puts the right side *into* the left side). Using `-` might seem
#' unintuitive if you think of the operator as "subtract", but look at it as a
#' hyphen instead (the underlying reason is that `-` is the only operator in the
#' same precedence group as `+`).
#'
#' Often you are interested in creating single column or single row layouts.
#' `patchwork` provides `|` (besides) and `/` (over) operators to support
#' stacking and packing of plots. See the exampels for their use.
#'
#' In order to reduce code repetition `patchwork` provides two operators for
#' adding ggplot elements (geoms, themes, facets, etc.) to multiple/all plots in
#' a patchwork. `*` will add the element to all plots in the current nesting
#' level, while `&` will recurse into nested patches.
#'
#' @param e1 A `ggplot` or `patchwork` object
#' @param e2 A `ggplot` or `patchwork` object in case of `/`, or a `gg` object
#' such as a geom or theme specification in case of `*` and `&`
#'
#' @return A `patchwork` object
#'
#' @name plot_arithmetic
#' @rdname plot_arithmetic
#'
#' @examples
#' library(ggplot2)
#'
#' p1 <- ggplot(mtcars) + geom_point(aes(mpg, disp))
#' p2 <- ggplot(mtcars) + geom_boxplot(aes(gear, disp, group = gear))
#' p3 <- ggplot(mtcars) + geom_bar(aes(gear)) + facet_wrap(~cyl)
#' p4 <- ggplot(mtcars) + geom_bar(aes(carb))
#'
#' # Standard addition vs division
#' p1 + p2 + p3 + plot_layout(ncol = 1)
#' p1 + p2 - p3 + plot_layout(ncol = 1)
#'
#' # Stacking and packing
#' (p1 | p2 | p3) /
#'       p4
#'
#' # Add elements to the same nesting level
#' (p1 + (p2 + p3) + p4 + plot_layout(ncol = 1)) * theme_bw()
#'
#' # Recurse into nested plots as well
#' (p1 + (p2 + p3) + p4 + plot_layout(ncol = 1)) & theme_bw()
#'
NULL

#' @importFrom grid is.grob
#' @rdname plot_arithmetic
#' @export
"-.ggplot" <- function(e1, e2) {
  if (is.grob(e2)) e2 <- wrap_elements(full = e2)
  if (!is.ggplot(e2)) stop("Only knows how to fold ggplot objects together", call. = FALSE)
  patchwork <- new_patchwork()
  if (is_patchwork(e2)) {
    plot <- plot_filler()
    patchwork$plots <- list(e1, e2)
  } else {
    plot <- e2
    patchwork$plots <- list(e1)
  }
  add_patches(plot, patchwork)
}
#' @importFrom grid is.grob
#' @rdname plot_arithmetic
#' @export
"/.ggplot" <- function(e1, e2) {
  if (is.grob(e2)) e2 <- wrap_elements(full = e2)
  if (!is_patchwork(e1)) {
    e1 + e2 + plot_layout(ncol = 1)
  } else if (!is.null(e1$patches$layout$ncol) && e1$patches$layout$ncol == 1) {
    e1 + e2
  } else {
    e1 - e2 + plot_layout(ncol = 1)
  }
}
#' @importFrom grid is.grob
#' @rdname plot_arithmetic
#' @export
"|.ggplot" <- function(e1, e2) {
  if (is.grob(e2)) e2 <- wrap_elements(full = e2)
  if (!is_patchwork(e1)) {
    e1 + e2 + plot_layout(nrow = 1)
  } else if (!is.null(e1$patches$layout$nrow) && e1$patches$layout$nrow == 1) {
    e1 + e2
  } else {
    e1 - e2 + plot_layout(nrow = 1)
  }
}
#' @rdname plot_arithmetic
#' @export
"*.gg" <- function(e1, e2) {
  if (is_patchwork(e1)) {
    e1$patches$plots <- lapply(e1$patches$plots, function(p) {
      if (!is_patchwork(p)) p <- p + e2
      p
    })
  }
  e1 + e2
}
#' @rdname plot_arithmetic
#' @importFrom ggplot2 is.theme
#' @export
"&.gg" <- function(e1, e2) {
  if (is_patchwork(e1)) {
    if (is.theme(e2)) {
      e1$patches$annotation$theme <- e1$patches$annotation$theme + e2
    }
    e1$patches$plots <- lapply(e1$patches$plots, function(p) {
      if (is_patchwork(p)) {
        p <- p & e2
      } else {
        p <- p + e2
      }
      p
    })
  }
  e1 + e2
}
